1. Field of the Invention
The present invention relates in general to the regulation of the output of a high frequency induction heating cooker, and more particularly to a circuit for compensating for the output of a high frequency induction heating cooker wherein the output is maintained at a constant value desired by the user although an input voltage is varied.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a block diagram of a conventional circuit for compensating for the output of a high frequency induction heating cooker. As shown in this drawing, the conventional output compensating circuit comprises a working coil W/C for generating a magnetic force to heat a conductive cooking container, a rectifying circuit 10 for rectifying an external alternating current (AC) input power into a direct current (DC) power, filtering the DC power and applying the filtered DC power to the working coil W/C, an input current detecting circuit 20 for detecting a voltage proportioned to current of the AC input power through a current transformer CT, and an output regulation signal generating circuit 30 for comparing the detected voltage from the input current detecting circuit 20 with an output regulation voltage Vref1 which is regulated by the user and generating an output regulation signal in accordance with the compared result.
The rectifying circuit 10 includes a bridge diode BD for rectifying the external AC input power into the DC power and a choke coil CH and a condenser C2 for cooperating to filter the DC power from the bridge diode BD and apply the filtered DC power to the working coil W/C.
The output regulation signal generating circuit 30 includes a first comparator 31 for comparing the detected voltage from the input current detecting circuit 20 with the output regulation voltage Vref1 and an integrator 32 for integrating an output signal from the first comparator 31 and outputting the integrated signal as the output regulation signal.
The conventional output compensating circuit also comprises a start driving circuit 40 for comparing the detected voltage from the input current detecting circuit 20 with a predetermined container discrimination voltage Vref2 and controlling the operation of the integrator 32 in the output regulation signal generating circuit 30 and generating a start drive voltage in accordance with the compared result, a triangular wave generating circuit 50 being triggered in response to a driven state of the working coil W/C to generate a triangular wave signal, and a switching circuit 60 for start-driving the working coil W/C in response to the start drive voltage from the start driving circuit 40, comparing the output regulation signal from the output regulation signal generating circuit 30 with the triangular wave signal from the triangular wave generating circuit 50 and switching the driving of the working coil W/C in accordance with the compared result.
The start driving circuit 40 includes a second comparator 41 for comparing the detected voltage from the input current detecting circuit 20 with the predetermined container discrimination voltage Vref2 and controlling the operation of the integrator 32 in the output regulation signal generating circuit 30 in accordance with the compared result and a start drive voltage generator 42 for generating the start drive voltage in response to an output signal from the second comparator 41.
The triangular wave generating circuit 50 includes a trigger circuit 51 for generating a trigger signal in response to the driven state of the working coil W/C and a triangular wave generator 52 for generating the triangular wave signal in response to the trigger signal from the trigger circuit 51.
The switching circuit 60 includes a third comparator 61 for comparing the output regulation signal from the output regulation signal generating circuit 30 with the triangular wave signal from the triangular wave generating circuit 50, a power transistor Q1 for switching the driving of the working coil W/C, a delay circuit 62 for delaying an output signal from the third comparator 61 by a predetermined time period, and a driver 63 for driving the power transistor Q1 in response to an output signal from the delay circuit 62.
A resonance condenser C1 is connected in parallel to the working coil W/C to resonate with the choke coil CH and a diode D1 is connected reversely and in parallel to the transistor Q1 to protect the transistor Q1.
The operation of the conventional output compensating circuit with the above-mentioned construction will hereinafter be described.
When the user turns on a power switch (not shown) of the high frequency induction heating cooker, the external AC power is supplied to the high frequency induction heating cooker. The external AC input power is full-wave rectified by the bridge diode BD and then filtered by the choke coil CH and the condenser C2. The filtered DC power is applied to the working coil W/C.
At this time, at an initial state, no current flows through the working coil W/C. As a result, the input current detecting circuit 20 outputs a zero voltage to inverting input terminals (-) of the first and second comparators 31 and 41. Then, a high signal is outputted from the second comparator 41, a non-inverting input terminal (+) of which is applied with the predetermined container discrimination voltage Vref2, thereby causing the integrator 32 to be disabled. Also, the start drive voltage is generated from the start drive voltage generator 42 and then applied to a non-inverting input terminal (+) of the third comparator 61, thereby causing a high signal to be outputted from the third comparator 61. The high signal from the third comparator 61 is applied to a base of the transistor Q1 through the delay circuit 62 and the driver 63. As a result, the transistor Q1 is turned on.
The turning-on of the transistor Q1 causes current to flow through the working coil W/C, resulting in the start driving of the working coil W/C. The current flowing through the working coil W/C is detected by the current transformer CT and the proportioned voltage is then detected by the input current detecting circuit 20.
On the other hand, the driving of the working coil W/C causes the trigger circuit 51 to generate the trigger signal. In response to the trigger signal from the trigger circuit 51, the triangular wave generator 52 generates the triangular wave signal, which is then applied to an inverting input terminal (-) of the third comparator 61.
At this time, the detected voltage from the input current detecting circuit, 20 is higher than the predetermined container discrimination voltage Vref2 and a low signal is thus outputted from the second comparator 41. As a result, the integrator 32 is enabled and the start drive voltage generator 42 is disabled. The detected voltage from the input current detecting circuit 20 is also compared in the first comparator 31 with the output regulation voltage Vref1 which is regulated by the user and a signal of a virtual ground level is thus outputted from the first comparator 31. The output signal from the first comparator 31 is integrated by the integrator 32 and then applied to the non-inverting input terminal (+) of the third comparator 61.
When the triangular wave signal from the triangular wave generator 52 is higher than the output signal from the integrator 32, a low signal is outputted from the third comparator 61 and then applied to the base of the transistor Q1 through the delay circuit 62 and the driver 63. As a result, the transistor Q1 is turned off.
Thereafter, when the triangular wave signal from the triangular wave generator 52 is lower than the output signal from the integrator 32, a high signal is outputted from the third comparator 61 and then applied to the base of the transistor Q1 through the delay circuit 62 and the driver 63. As a result, the transistor Q1 is turned on.
The turning-on/off of the transistor Q1 are controlled repeatedly in the above-mentioned manner, resulting in generation of the magnetic force in the working coil W/C. The surface resistance of the conductive container becomes large due to the magnetic force from the working coil W/C, so that heat is generated from the conductive container. In result, the cooking of food is performed by the heat.
However, the conventional circuit for compensating for the output of the high frequency induction heating cooker has a disadvantage in that it is controlled by the detected voltage from the input current detecting circuit and the output regulation voltage which is regulated by the user. Namely, since the drive voltage to the working coil is varied as a variation in the AC input power voltage, the output of the high frequency induction heating cooker is varied. For this reason, the output of the high frequency induction heating cooker cannot be controlled at a constant value desired by the user.